154 
AMERICAN JOURNAL OF BOTANY 
[Vol. io. 
plane of the thallus, intersecting the outer exposed wall of the cell and 
sloping upward interiorly (figs. 2, 9, 13). In some of the cases observed 
the plane of division is more nearly horizontal than indicated in figures 2 
and 9. It is clear that, if seen in horizontal sections, the secondary seg¬ 
ments shown in figure 2 and those in process of formation in figure 9 would 
appear to be “inner posterior” and “outer anterior” cells; this is the case 
in the horizontal section shown in figure 1, in which c and b' are the daughter 
cells in question. Miss Clapp figures only the horizontal sections (see her 
figures 3-10), and I am unable to determine with certainty from these 
whether in her material the division of the primary segment is a vertical 
transverse one, or whether, as I have found in harmony with Leitgeb, it is 
really in a diagonal plane. 
Campbell says that “the segment first divides into an inner and an 
outer cell, and the former probably next into a dorsal and a ventral one.” 
He does not figure these divisions, and it is not clear to me whether he means 
that the first division of the segment is into an inner posterior and an outer 
anterior cell, or into an inner cell proximal to the apical cell, and an outer cell 
distal to it. His description of the origin of the sex organs seems to imply 
that he means the latter, but it is difficult to reconcile his statement with 
the observations of Leitgeb, or with those recorded in the present paper. 
As described by Miss Clapp, the apical cell is located in a sinus resulting 
from the forward growth of the thallus to the right and left of this cell. 
Each sinus contains usually two or more apical cells, which gradually di¬ 
verge as new segments are cut off and undergo further division. As the 
two apical cells thus diverge, the group of cells between them grows forward 
and divides the sinus into two; but in the meantime each apical cell usually 
has given rise to another, so that each new sinus contains two apical cells. 
The time of the formation of new apical cells seems to vary, however, so 
that a sinus may contain more than two or (rarely) only one apical cell. 
The branching is truly dichotomous, but usually one branch of each pair 
develops only slightly and thus becomes apparently lateral (in this con¬ 
nection see also Clapp, 1912, and Campbell, 1905). During the season 
of gamete production these lateral branches begin very early to produce 
sexual organs, thus becoming gametophores. Text figure 3 shows a series 
of transverse sections, 10 ix in thickness, of the growing end of a thallus 
having four apical cells (a) in one sinus. The portion of the thallus in 
the middle of the sinus had just begun to push forward in advance of the 
apical cells, and two young archegonia ($ ) shown in photographs A and B 
indicate that the two apical cells uppermost in the photographs were initi¬ 
ating an archegonial branch. The apical cell and the young cells immedi¬ 
ately adjacent to it are much larger than the older cells a little farther 
removed from the apical cell and contain relatively few plastids. In the 
haematoxylin preparations the cytoplasmic portions of these cells are 
stained only lightly (text fig. 3). 
